C&EN: TODAY'S HEADLINES - FROM CRYSTAL TO CRYSTAL EXPERTLY

MAUREEN ROUHI

Predicting the crystalline form of molecular solids is largely an empirical exercise. Now a strategy to prepare molecular crystals with perfectly predictable architecture has been demonstrated by a group led by James D. Wuest, a chemistry professor at the University of Montreal.


	REACTIVE Allylic sites in porous molecular crystal may be photochemically functionalized with monothiols or cross-linked with dithiols. IMAGE CREATED BY CHRISTIAN GRAVEL, UNIVERSITY OF MONTREAL

Predictable crystalline architecture can be achieved by starting with permeable molecular crystals with reactive sites in their porous interiors that reagents can reach through diffusion, Wuest says. A reaction that maintains the crystalline architecture will then yield a product of known crystalline structure but different molecular composition.

Wuest and coworkers have built porous molecular crystal networks from a tetraphenylmethane modified to incorporate multiple hydrogen-bonding sites and reactive allylic sites [Angew. Chem. Int. Ed., published online Sept. 19, http://www3.interscience.wiley.com/cgi-bin/fulltext/105558603/PDFSTART]. The molecules crystallize from dioxane as a hydrogen-bonded tetragonal network with helical channels about 7.8 Å wide and 11.4 Å high. Years of work went into figuring out what kind of building blocks will assemble into such porous networks, Wuest says.

When the dioxane in the channels is replaced with toluene, the crystals are otherwise unchanged. When the allyl groups are made to react with thiols of suitable size (methane-, ethane-, and propanethiol) by irradiation, up to 85% of the sites react, but the product maintains the architecture of the reactant crystal. And when the allyl groups are made to react with dithiols (ethane- or propanedithiol), the crystal architecture again remains the same.

But compared to the starting crystal, the product is more robust--indifferent to hot aqueous hydrochloric acid or trifluoroacetic acid and stable up to 200 °C. X-ray and spectroscopic studies reveal that the hydrogen-bonded network has become a covalently cross-linked, porous, single-crystalline macromolecule. The strategy may lead to robust organic zeolites, Wuest says.

The Wuest group's porous networks are "among the most open and robust for simple organic compounds, and their ability to functionalize the interior of a crystalline molecular solid is unprecedented for small-molecule crystals," comments Miguel A. Garcia-Garibay, a chemistry professor at the University of California, Los Angeles. "The formation of a covalently cross-linked solid by using diffusing reagents is the first example that I am aware of," he adds.

John R. Scheffer, a chemistry professor at the University of British Columbia, Vancouver, notes that reactions forming single-crystal products from single-crystal starting materials--or topotactic reactions--often are discovered accidentally. Given the importance of the solid state in materials science, "being able to control crystals is a major area of research," he says. The Wuest group's technique is a "first general step to planned topotactic reactions," he adds.